The present invention generally relates to drop-out compensation apparatus, and more specifically, to apparatus for providing drop-out compensation in a digital data stream having reoccurring intervals of similar information.
A magnetic recording and reproducing apparatus is disclosed in our copending application Ser. No. 117,428, which is of the type which records tracks substantially transversely relative to the longitudinal direction of the tape, but which does not utilize a FM recording technique that is currently most widely used in commercial quality television broadcasting. The apparatus disclosed therein records and reproduces pulse code modulated digital signals and offers many significant advantages compared to FM recording and reproducing apparatus. While the apparatus disclosed therein offers significantly improved overall performance compared to commercial quality television signal FM recording apparatus, an occasional loss of the recovered signal during reproducing occurs in magnetic tape recording systems, due to tape irregularities such as scratches and surface roughness and the like which cause drop-outs in the recovered signal. The apparatus of the present invention provides compensation for such drop-outs in that substitute digital data is inserted into the data stream in place of data that is detected to be either defective for some reason, wherein defective data as used herein may include incorrect or missing data.
While drop-out compensators, as they are known in the art, have been in existence for some time, they generally merely insert substitute signals that occurred a short time prior to the occurrence of the drop-out and hopefully provide a value that has a high probability of being visually equivalent to the defective signal. However, by utilizing a substitute data value that is horizontally displaced relative to the missing information in a video raster, the resulting image can be easily detrimentally affected, particularly if the preceding-in-time substitute data had a much different intensity, as would occur on opposite sides of a sharp vertical line, e.g., the edge of a building where the left side of the vertical edge is light and the right side dark. In such case, replacing the data on the dark side with preceding-in-time data that is light in intensity provides no improvement in the resulting image that is viewed on the raster. Similarly, by using data from a few preceding horizontal lines having the same chroma phase relationship as the data being replaced, wherein the data is at the same relative horizontal location along the line, similar effects are produced if a significant intensity transition occurs in the vertical direction of the image represented by the data. Moreover, by automatically inserting the data from a lateral location in the same horizontal line or from the same relative lateral position from a correspondingly phased preceding line, there is the possibility that defective data will be substituted, since there is no assurance that the automatically substituted data is not itself defective.